1. Field of the Invention
The present invention relates to a method and an apparatus for inspecting leakage of a container which utilizes gas to detect the leakage.
2. Description of the Related Art
Leakage inspection is necessary in a manufacturing process or a recycling (recovering) process of containers such as drum cans and fuel tanks. For example, for the drum can for storing liquid without the leakage, apertures of 50 μm or more must be detected. In this case, even if there are apertures lower than 50 μm, it can be assumed that any leakage does not practically occur from the container.
As a leakage inspection method of the container, there have conventionally been known an immersion method and a soap water method. Hereinafter, the immersion method and the soap water method will be described.
First, the immersion method will be described. (1) At the beginning, gas (e.g., air) is injected into a container in a sealing state to pressurize the container. (2) Then, the pressurized container is immersed in liquid (e.g., water). (3) If there is an aperture in the container, the gas (e.g., air) leaks through the aperture from the inside to generate bubbles on a surface of the container in which the aperture is present. (4) The bubbles are visually searched. If the bubbles are identified, it is determined that the container has an aperture to cause leakage.
Next, the soap water method will be described. (1) First, a container is hermetically sealed, and gas (e.g., air) is injected inside to pressurize the container. (2) Then, soap water is coated on a surface of the container. (3) If there is an aperture in the container, the gas (e.g., air) leaks through the aperture from the inside to generate soap bubbles on the surface of the container in which the aperture is present. (4) The soap bubbles are visually searched. If the soap bubbles are identified, it is determined that the container has an aperture to cause leakage.
Conventionally, the aperture which causes the leakage of the container has been visually inspected by the aforementioned methods. It is theoretically conceivable that the immersion method and the soap water method can discover apertures larger than 50 μm. However, since both of the methods employ visual inspection by skilled workers, there has been uncertainty over reliability of detection of apertures in a range of 50 μm to 100 μm. In other words, bubbles or soap bubbles caused by leakage through the apertures of 50 μm to 100 μm are so small that they are overlooked in the visual inspection.
Additionally, in the case of inspecting the leakage of the container by the immersion method, a drying step is necessary to remove liquid from the surface after the process of the leakage inspection. In the case of the soap water method, cleaning and drying steps are necessary to remove soap water from the surface after the process of the leakage inspection.
On the other hand, in a leakage inspection technology field of a vacuum chamber, there has conventionally been known a leakage inspection method which uses helium gas. However, the leakage inspection method which uses the helium gas is improper for leakage inspection of a container such as a drum can or a fuel tank because sensitivity is too high. In other words, the helium gas has behavior that it leaks though an aperture much smaller than 50 μm, so that a detector detects the helium gas leaked through the gap of a sealed cap or the like of the container, and a presence of apertures of 50 μm to 100 μm which should be found cannot be confirmed.
As described above, in the conventional immersion and soap water methods, as they are visual inspection methods, the reliability of discovery of apertures of small sizes (e.g., 50 μm to 100 μm) is low, and the necessity of removal of water, soap or the like after the leakage inspection requires time and cost in the post process. Additionally, in the leakage inspection method which uses the helium gas, presence of an aperture of a target size cannot be identified because the sensitivity is too high.
Therefore, there has been a strong desire for a highly reliable method and a highly reliable apparatus for inspecting leakage through a narrow aperture in place of the conventional immersion and soap water methods. Moreover, there has been a strong desire for a leakage detection method and a leakage inspection apparatus of a container which need no post process.